1. Field of the Invention
This invention relates to a power supply system including a fuel cell.
2. Description of Related Art
A fuel cell in general has a membrane electrode assembly (MEA) including a pair of electrodes (formed of an anode and a cathode) holding the opposite sides of an electrolyte membrane, and a pair of fuel cell separators holding the opposite sides of the MEA. The anode has an anode catalyst layer and a diffusion layer. The cathode has a cathode catalyst layer and a diffusion layer. When the fuel cell is generating power, hydrogen gas serving as anode gas is supplied to the anode and oxygen gas serving as cathode gas is supplied to the cathode, whereby reaction that produces hydrogen ions and electrons takes place at the anode. The hydrogen ions reach the cathode through the electrolyte membrane, while the electrons reach the cathode through an external circuit. On the other hand, reaction among hydrogen ions, electrons and oxygen gas takes place to produce water at the cathode, whereby energy is emitted.
Recently, researches, developments and approaches have been widely conducted to utilize a fuel cell powered vehicle or the like having a fuel cell mounted as a driving energy source, as a source of electricity (external power source) for external loads. Japanese Patent Application Publication No. 2004-187385 (JP 2004-187385 A) for example proposes a distributed power supply system including a fuel cell powered vehicle having a fuel cell as a driving power supply, and an external power network which receives electric power generated by the fuel cell and outputs the electric power to external loads. This distributed power supply system is configured such that an on-vehicle fuel cell is operated to generate electric power and this generated power is supplied externally when the fuel cell powered vehicle is parked.
The distributed power supply system described in JP 2004-187385 A is configured on the assumption that the fuel cell is operated to generate power under maximum efficiency conditions during nighttime when the fuel cell powered vehicle is parked for a long period of time, and the generated power is output to a commercial power system that is an external power network. However, the range of applications of such a fuel cell mounted on a fuel cell powered vehicle used as an external power source is not limited to a commercial power system. The on-vehicle fuel cell is rather required to function as an external power source to directly supply power to electric appliances in a household, emergency shelter, and restoration site when the power supply from a commercial power system (commercial power source) is lost at a time of disaster or the like, for example.
Electric power required to operate electric appliances is usually several kW at most, whereas a fuel cell for driving a vehicle is able to generate electric power as high as from slightly less than 10 kW to about 100 kW or more (stack output) when it is operated normally. This means that a large amount of excessive power is generated when such power generation capable of generating power as high as about 100 kW is performed in order to supply power to an external load of about several kW since no power is required to drive accessories required to operate the vehicle.